With Lightning -- and Aura -- Salesforce emphasizes how users can design applications that not only look great, but plug into more than Salesforce. Where, then, does Salesforce's open source offering end with Aura, and where do its own services begin?

The front end is free

Aura has been brewing since 2013, with the core framework available under the Apache license. The open source pieces cover two items: a Java server for the back end, and a set of UI components.

Salesforce

Aura UI components, as deployed in a Salesforce Lightning app.

Adam Seligman, Salesforce SVP of Developer Relations, compared this stack to frameworks like Ember or React: "The idea was to take the best ideas in the rapidly evolving [JavaScript] framework space, add what's needed for trust in the enterprise, and use the power of our back-end platform to optimize performance and security."

What is Aura's relationship to Lightning? Seligman put it this way: Lightning is "[an implementation of] Aura on the Salesforce platform," with Aura on the front end and Salesforce's APIs and data access on the back end. The Lightning Design System, which lets you create Aura apps that have a consistent Salesforce look and feel, is also available as an open source project.

Aura contains "features and components that are not currently available in the Lightning Component framework," according to the developer's documentation -- mainly tabs, modals, lists, and trees. (Salesforce is "working to surface more of these features and components for Salesforce developers.")

Want app building tools? That'll cost you

Aside from integrating with Salesforce data, Salesforce's design tools are an advertised draw for using Aura directly on Salesforce. Lightning App Builder allows Lightning components to be dragged, dropped, and hooked up to a variety of data sources.

You can't do the actual drag-and-drop creation in App Builder with Aura alone; for that, you must be plugged into Salesforce. But apps designed this way can connect with a number of data sources apart from Salesforce -- and Aura applications can be built apart from the App Builder, albeit by hand.

Even with its design tools, Aura will have to go up against the existing ecosystem of software to win broad appeal among developers working on UIs for business apps. To combat this perception, Salesforce is positioning Aura and the library of Lightning components (available through Salesforce's online AppExchange) as more than a repurposing of current open source tools.

"There is a lot of [Web UI] framework innovation," said Seligman, "but we need something powerful enough for developers while also being stable enough for 150,000 customers and millions of users."